The present invention generally relates to predistortion methods, in particular, predistortion methods used in conjunction with amplifiers.
Current wideband code division multiple access (W-CDMA) third generation (3G) systems will likely form an infrastructure for mobile speech, video, and high-speed data communications. The number of broadband Internet services is rapidly increasing in fixed networks, and people accustomed to having a broadband environment in their home are now beginning to expect a similarly broadband mobile environment. As a result, the 3G systems are evolving to accommodate more subscribers and provide broadband mobile data communications.
Networks and base transceiver stations (BTSs) for third generation systems have been deployed; however, these BTSs do not have the capabilities needed to provide full 3G mobile services. The next phase of equipment should support greater capacities and faster data services such as high-speed downlink packet access (HSDPA) for W-CDMA and CDMA2000 systems. However, total power consumption of high-capacity BTSs that are required tends to be higher than that of current lower capacity BTSs, because the high-capacity BTSs use more radio frequency (RF) carriers and have more baseband signal processing units. This increased power consumption makes it difficult to implement the high-capacity BTSs using current BTS infrastructure. In particular, the heat produced by the power amplifiers implemented in high-capacity BTSs overwhelms the heat removal capacity of the current BTS infrastructure. Increasing the linearity of the power amplifier used in high-capacity BTSs may allow their use in the current BTS infrastructure.
Ideally, an amplifier provides uniform gain throughout a dynamic range thereof so that the output signal of the amplifier is a correct, amplified version of an input signal. However, in reality, amplifiers exhibit non-ideal properties such as non-linear amplitude and phase distortion, which are undesirable and may deteriorate performance of a system employing the amplifier.
One effect of this is the generation of output frequencies equal to sums and differences of integer multiples of input frequency components. This effect is known as intermodulation distortion (IMD) and is particularly undesirable in high-power radio frequency (RF) amplifiers designed for use in multicarrier or multichannel systems. For example, a broadband amplifier used in a wireless system may generate various undesirable intermodulation products as a result of amplifying a multitude of channels occurring at fixed frequency intervals across a band.
In order to compensate for the non-linearity of an amplifier, the amplifier may be operated in a linear zone. That is, the lower the power level of the amplifier, the smaller the non-linearity manifested by the amplifier. However, this may unnecessarily limit the acceptable operating range of the amplifier, since the amplifier must be operated below maximum power output to avoid undesirable non-linearity.
Another possible linearization method includes using a testing stage applied to an amplifier prior to a field implementation thereof. During the prior testing stage, a test signal may be amplified, a corresponding output signal may be sampled at a fast rate over a short period, and the input signal may be compared with the sampled output signal so as to determine distortion parameters specific to the amplifier at the time the sampling was performed. These distortion parameters, also known as coefficients, may be used to modify an input signal of the amplifier such that an output therefrom is as linear as possible. This technique for compensating for the non-linearity of an amplifier does not take into account how the amplifier's physical operational state may change as the amplifier ages in the field. Moreover, the determined distortion parameters may not be optimum for various amplifier input signals.